There exist various kinds of self-propelled crushing machines for crushing rocks, for crushing concrete, for crushing woods and the like. For example, the self-propelled crushing machine for crushing woods has, as shown in FIGS. 1 and 2, a rotary crusher 1 and a rotary tub 3 which introduces a wood 2 thrown from an outer portion to the rotary crusher 1 due to a rotation on a self-propelled truck 4, crushes the wood 2 introduced from a rotary tub 3 to the rotary crusher 1 and discharges to the outer portion. The details are as follows.
The rotary crusher 1 is a so-called hammer mill. This has a plurality of cutters 1b on an outer periphery of a shaft 1a which is structured to be made rotatable by crusher drive means, and crushes the wood 2 by the cutters 1b. The crusher drive means is driven by an oil hydraulic pressure, directly driven or the like.
The rotary tub 3 has a funnel 3b which is made rotatable by the tub drive means on a fixed bottom plate 3a. A part of the fixed bottom plate 3a is open, and the cutters 1b of the rotary crusher 1 can be overviewed from the opening. The tub drive means is also of an oil hydraulic driven type, a direct driven type or the like.
When throwing the wood 2 having a long size into the rotary tub 3, a lower end of the wood 2 is brought into contact with the upper portion of the fixed bottom plate 3a and the cutters 1b within the opening. On the contrary, the wood 2 falls down and an upper side surface thereof is brought into contact with an inner wall of the funnel 3b. A plurality of convex portions are provided on the inner wall of the funnel 3b in a vertical direction, and the convex portion presses the wood 2 due to a rotation of the funnel 3b. As a result, the lower end of the wood 2 reciprocates between the upper portion of the fixed bottom plate 3a and the cutter 1b while the wood 2 changes an attitude thereof, so that even the long wood 2 can be crushed by the cutters 1b. The crushed wood 2 is used for a pulp raw material, a manure, a fuel and the like.
In this case, the self-propelled crushing machine is structured such that when the raw materials are large or hard, or when they are mixed with the small raw materials or the soft raw materials, a load of the crusher is increased, a rotational speed is reduced, and a crushing efficiency is lowered. The reduction of the rotational speed causes a breakage of the crusher. Then, there is a structure made such as to automatically stop a raw material supply apparatus (the rotary tub 3 in the case of being used for crushing the wood) when the rotational speed of the crusher is lowered to a predetermined value Nb, and to automatically start the raw material supply apparatus when the rotational speed of the crusher is inversely increased to a predetermined value Na. In this case, in order to prevent the automatic stop and the automatic start from generating a hunting, a relation between the predetermined values Na and Nb is set to, for example, a relation Na&gt;Nb+50 rpm.
However, the prior art mentioned above has the following problems.
(1) Since the raw material supply apparatus is automatically started or automatically stopped in accordance with a rotational change of the crusher, there is no function for automatically returning the crusher to a normal rotation although the reason of breaking the crusher is solved. Accordingly, a reduction of a crushing efficiency is unavoidable. PA1 (2) A crushed grain size (a piece size in the case of the crusher for crushing the wood) becomes finer as the rotational speed of the crusher becomes higher. If the normal rotational speed of the crusher is set to Ns, a relation Ns&gt;Na and Ns&gt;Nb is established. In this case, since the relation Na&gt;Nb is established as mentioned above, the changing range of the rotational speed of the crusher becomes wide to Ns to Nb. Accordingly, it is hard to obtain the crushed material having a fixed grain size. PA1 (3) In particular, the self-propelled crushing machine for crushing the wood has the rotary tab, however, there has not been suggested a technique structured such as to preferably control the rotational speed and perform a crushing having a higher efficiency. PA1 (4) The start and stop of the rotary tub depend only upon the rotational change of the crusher, and the rotary tub itself does not have an automatic control function. Further, the breakage of the crusher is indirectly prevented by the automatic start and the automatic stop of the rotary tub, and the crusher itself does not have an automatic control function. PA1 (5) For example, when a long member made of a wood 2 and the like is held between the convex portion of the rotary tub 3 and the cutters 1b, a rotational force of the rotary tub 3 pushes the cutter via the wood 2. Accordingly, an excess load is generated in the rotary crusher 1, and the rotational speed is suddenly reduced. However, in the self-propelled crushing machine for crushing the wood 2, there is an operational effect that the rotary tub 3 is further rotated, so that the nipped wood 2 is taken out and the crushing is again started. In the case that the predetermined value Na is set so as to automatically stop the rotary tub 3, as in the prior art, this operational effect can not expected. On the contrary, when the thick and hard wood 2 is completely meshed with the cutters 1b, it is impossible to discharge the meshed wood 2 only by the automatic stop of the rotary tub 3 as in the prior art and it is necessary to discharge the wood 2 by human hands, so that the crushing efficiency is bad.